The present embodiments relate to a transmitting device for driving a high-frequency antenna of a magnetic-resonance device using an amplitude-modulated target signal.
Known magnetic-resonance devices are employed for clinical applications. As part of the imaging process, nuclear spins of an object being examined that have been aligned by the magnetic-resonance device via a high-frequency antenna are excited, and image data is registered using a receiving antenna. For operating the transmitting antenna, a high-frequency signal (e.g., a target signal) that is capable of being amplitude-modulated and has a high power is used. The high powers may be supplied by a power-amplifier unit incorporated in a transmitting unit of the magnetic-resonance device.
The amplitude modulation of the high-power transmission signal is problematic. The target signals employed for exciting are generated by linearly operating transistor power amplifiers. The theoretically possible efficiency of the Class-B amplifiers customarily employed is at most approximately 78%. However, this efficiency drops sharply when the possible output voltage (amplitude) is exploited partially. The result is that small output voltages will give rise to substantial power dissipation that drives up the cost of the power semiconductors and of cooling the power semiconductors. Owing to the given technical limits in the amplifier, the mean amplifier power that is possible may not always be exploitable in view of the specific absorption rate (SAR) in the patient.
Another problem is that the power-amplifier unit is matched to a specific load impedance of the high-frequency antenna. The impedance experienced by the respective amplifier module may vary owing to changes in the load due to the object being examined (e.g., a patient) or owing to couplings between a plurality of high-frequency antennas. There is a drop in efficiency in the case of a low-ohmic load (large current, small voltage), and the output power that may be drawn has to be reduced.
Because the basic problem described occurs not only in the magnetic resonance but whenever amplitude-modulated high-frequency signals are to be generated (e.g., in broadcasting or mobile radio applications), there are already a number of approaches to a solution that are described in, for example, a series of articles by Frederick H. Raab et al. that have been published in succeeding editions of “High Frequency Electronics,” starting with part 1 in the issue dated May 2003, pp. 22-36. However, all the approaches described there have various disadvantages.
Thus it was proposed to use a Doherty modulation or Chireix modulation. Only a moderate efficiency gain may be achieved thereby in a limited amplitude range. It was further proposed to modulate the target amplitude using a variable operating voltage (e.g., Kahn concept or EER concept). An external fast modulator is disadvantageously required for that. A switching amplifier with direct delta-sigma modulation may also be used, although high-level switching losses occur owing to frequent edges, and extremely broadband end-level controlling is necessary. Output filtering is also required.
In the fifth part of the series of articles (“High Frequency Electronics,” January 2004, pp. 46-54), subjecting the carrier signal to pulse-width modulating was proposed. The width of each “burst” is proportional to the wanted envelope of the output amplitude. A bandpass filter is used for generating the amplitude-modulated target signal from the output signal of the amplifier. However, disruptions and efficiency losses disadvantageously occur owing to powers in sidebands. A narrowband output filter is used, which is expensive and space consuming.